There are many instances in which it is desirable to run an electrical power cable from a location at which a power source is located to a remote location. For example, this becomes necessary when utilizing AC power to operate portable power tools at a location remote from a building. Similarly, it is desirable to be capable of charging an electrical vehicle by running an electrical cable from a power source, such as an AC wall outlet to the location of the vehicle.
It has been a common practice to provide personnel protection devices against accidental electric shock in such cables in the case of a ground fault. Typically, such ground fault protection involves differential sensing of a current flow in the power lead and the return lead in the cable and disconnecting the power to the cable when the difference (an imbalance) exceeds a predetermined threshold value. Ground fault protection is effective to avoid electric shock when a person who is grounded accidently touches the power lead or the return lead. Resulting current flowing through the person will flow through one of these leads, but not the other, thereby causing tripping of the protection system, and protecting the individual against electrocution by turning off the power to the cable.
One form of personnel protection is particularly desirable. That is, to assure that no power is connected to the cable, unless the cable is connected to a utilization or load device. Various forms of mechanical interlocks have been utilized to make power at the remote end of the cable inaccessible, unless the cable is connected to a utilization device. However, such interlocks suffer from relatively low reliability, in that any mechanical failure of the interlock is likely to result in electric shock to an individual. Furthermore, the mechanical interlocks and their modes of operation are entirely visible and apparent to a user. Accordingly, mechanical interlocks are particularly vulnerable to being defeated by persons who simply do not want to be inconvenienced by the interlocks. This is often another source of personnel injury.
Broadly, it is an object of the present invention to avoid the shortcomings of prior personnel protection systems for electrical connections. It is specifically an object of the present invention to provide a personnel protection system in which power at the outlet of the cable is disconnected, unless the cable is connected to a utilization device.
It is another object of the present invention to provide a personnel protection system for power connections which disconnects power in the cable when the cable is not connected to a utilization device and which does not depend on mechanical interlocks.
It is another object of the invention to provide a system that will not furnish continuous electrical power unless safe conditions for receiving the power are fully satisfied.
It is yet another object of the present invention to provide a personnel protection system for electrical power connections which is readily retro-fitted in existing personnel protection systems.
It is yet another object of the present invention to provide a personnel protection system for electrical connections for disconnecting power to a cable when it is not connected to a utilization device, which system is convenient and reliable in use, not readily defeated, yet relatively simple and inexpensive in construction.
In accordance with the present invention, a personnel protection system for electrical connections which is of the type that differentially senses the current flow between the power lead and the return lead (or between two power leads) is provided with an unbalancing impedance which makes the current difference between the two leads sufficiently high to trip the device and cause the power to be disconnected. A utilization device is similarly provided with an imbalanced impedance. However, that impedance is calculated to counteract the current imbalance caused by the first impedance, so that when the cable is connected to the utilization device, the current balance between the two conductors is restored, and power connection is maintained in the cable. In addition, the current flow in the second impedance is sensed and, if it is not at the correct level, electrical power to the load is disconnected in the same manner as when an imbalance occurs. Thus, the system does not provide continuous power unless predetermined conditions considered safe for securing the power are fully satisfied.